This invention relates to a method for producing a bladder for use in a tire forming process where a green tire is expandingly formed and vulcanized in a toroidal shape.
Generally, a green tire is placed in a vulcanizing apparatus having upper and lower molding members, and expandingly formed into a toroidal shape. Thereafter, the green tire is vulcanized by heating inner and outer portions of the green tire.
More specifically, the vulcanizing apparatus is provided with a central mechanism for supporting a green tire. The upper and lower molding members are disposed around the central mechanism to mold the green tire into a specified shape. The central mechanism includes a bladder in the form of a rubber bag for expanding the green tire into a toroidal shape.
After mounted on the central mechanism of the vulcanizing apparatus, the green tire is applied with the vulcanizing operation. In the vulcanizing process, vulcanizing medium is supplied into the bladder at a high temperature and high pressure to expand the green tire into a toroidal shape. After a predetermined time, the expanding green tire is restricted by the upper and lower molding members, and kept in a specified toroidal shape. After the vulcanization of the green tire is completed, the upper and lower molding members are separated and the bladder is contracted to enable the vulcanized tire to be removed from the vulcanizing apparatus.
In the vulcanizing apparatus, the bladder is made in close contact with the inner circumferential surface of the green tire for a long time under the conditions of high temperature and high pressure. Accordingly, there is a likelihood that the bladder adheres to the tire. This will make it difficult to remove the vulcanized tire from the bladder of the vulcanizing apparatus.
In order to solve this problem, recently, there has been proposed a measure of coating a separating agent layer on an outer surface of the bladder to assure easy separation between the bladder and the vulcanized tire. However, there is a likelihood that oil components ooze from the bladder and spread in between the outer surface of the bladder and the separating agent layer. As a result, the separating agent layer is liable to peel off from the outer surface of the bladder and the durability of the bladder consequently decreases.
The oil components causing the separating agent layer peeling are softener and plasticizer, such as castor oil, which had been added to crude rubber in the kneading process of the bladder production. To prevent such peeling, there has been proposed a way of reducing the amount of softener and plasticizer in the kneading process. However, the amount of softener and plasticizer is closely related to the workability or the ability of shaping of crude rubber. Accordingly, the reducible amount of softener and plasticizer is limited. Thus, this way has not been sufficiently feasible.